This invention relates to a reheating device to be used with a steam power plant, and more particularly to a reheating device of the type adapted to prevent subcooling of condensate created at the outlet ends of U-shaped heat exchanger tubes.
In an ordinary nuclear power plant of the type of boiling water reactor or pressurized water reactor, steam supplied to the steam turbine is far wetter than that of fossil power plants. Because of the corrosion of the turbine blades and an adverse effect on the thermal efficiency, moisture content must be removed out of the wet steam. The removal of the moisture content is ordinarily realized by use of a moisture separator of, for instance, a chevron type, having corrugated plates with drain pockets, which is provided between a high-pressure turbine and a low-pressure turbine of the nuclear power plant, so that the moisture content of about 10% contained in the steam exhausted from the high-pressure turbine is reduced to less than 1%. The steam with the moisture content thus reduced is then reheated in a reheating device by means of steam extracted from the high-pressure turbine or steam generated from the nuclear reactor in a reheating cycle of the turbine operation for improving the thermal efficiency and protecting turbine blades from corrosion. The reheater and the moisture separator are ordinarily provided commonly in the shell, the combined device being ordinarily termed a moisture separator reheater.
Two types of moisture separator reheater are available for such purposes, one being a single stage type heated by the steam generated from the nuclear reactor, and the other being a two stage type heated firstly by the steam extracted from the high-pressure turbine, and secondly by the steam generated from the nuclear reactor. Both types of the reheating devices are constructed in the form of a multitube type heat exchanger wherein high temperature heating steam flows inside of the tubes, while the steam to be reheated flows outside of the tubes.
FIGS. 1 through 4 illustrate a conventional two-stage type reheating device combined with a moisture separator. The moisture separator reheater comprises a shell 1 of a horizontally extending cylindrical configuration. Two steam inlet pipes 2 and one drain exhaust pipe 3 are connected to lower portions of the shell 1, to the upper portions of which two steam outlet pipes 4 are connected. End plates 5 enclose both longitudinal ends of the shell 1 entirely, and internally of the end plates 5, there are provided two partition plates 6 which extend vertically so as to separate the interior of the shell 1 into different portions.
As illustrated in FIG. 2 clearly, the conventional device further comprises a bottom plate 7 provided in a lower part of the shell 1 to extend horizontally among the partition plates 6, a ceiling plate 8 provided above the bottom plate 7 to extend in parallel with the bottom plate 7, and two steam distributing plates 9 extending between the bottom plate 7 and the ceiling plate 8 obliquely upwardly so as to form a steam distributing chamber 10 of a triangular cross-section on the bottom plate 7. Moisture separating devices 11 are further provided laterally outwardly of the steam distributing plates 9 between the bottom plate 7 and the ceiling plate 8 for separating the moisture content out of the wet steam introduced into the steam distributing chamber 10.
Two plates 12 are further extended obliquely upwardly from the lateral edges of the ceiling plate 8 so that the upper edges of the plates 12 are combined together in an angular relation. Outwardly of the plates 12, plates 13 are further provided in parallel with the two plates 12, so that two steam reheating passages 14 are formed between the plates 12 and 13, respectively. The upper ends of the steam reheating passages 14 are combined into a single passage connected to the steam outlet pipes 4. In each of the steam reheating passages 14, heat exchanger tubes 17 bent into U-shape are provided.
In the spaces formed between the end plates 5 and the partition plates 6 provided at both ends of the shell 1, there are provided a first header 15 and a second header 16 for the first and second steam reheating devices, respectively. As best illustrated in FIGS. 3 and 4, each of headers 15 and 16 is divided by a partition wall 18 for separating the pass into a high-temperature chamber 19 and a low-temperature chamber 20, and the aforementioned U-shaped heat-exchanger tubes 17 are provided so that the upstream ends thereof open in the high-temperature chamber 19, while the downstream ends thereof open in the low-temperature chamber 20. A tube plate 26 has holes, not shown, in which the upstream and downstream ends of the U-shaped tubes 17 are tightly received, and a plurality of support plates 21 for supporting the heat-exchanger tubes 17 in a spaced apart relation are provided in each of the steam reheating devices, so that the support plates 21 prevent the tubes 17 from vibrations and the like. A heating steam inlet pipe 22 is connected with the high-temperature chamber 19, while a drain exhaust pipe 23 and a vent steam outlet pipe 24 are connected with the low-temperature chamber 20. A manhole 25 is further provided in the low-temperature chamber 20.
In the above described moisture separator reheater of conventional construction, the steam to be reheated supplied through the steam inlet pipe 2 flows in the steam distributing chamber 10 in the shell 1 and is then divided by the steam distributing plates 9 into two parts. The steam then flows through the moisture separating devices 11, each having corrugated plates with drain pockets, which remove the moisture content out of the steam. The moisture content (or drain) thus removed from the steam flows downwardly out of the device 11 by a gravitational force, and is exhausted through the drain exhaust pipe 3 into a drain tank, not shown. The steam passed through the moisture separating devices 11 is guided to flow through the steam reheating passages 14 in the first and second steam reheating devices. While the steam passes between the U-shaped heat exchanger tubes 17 of the steam reheating devices, the steam is heated by the heating steam flowing inside of the heat-exchanger tubes 17, and the steam thus heated into a superheated condition is then sent through the two steam outlet pipes 4 into the low-pressure turbine.
On the other hand, the heating steam extracted from the high-pressure turbine or received from the nuclear reactor is introduced through the heating steam inlet pipe 22 into the high temperature chamber 19 of the header 15 or 16. The heating steam is then caused to flow through the U-shaped heat exchanger tubes 17. In the tubes 17, the heat of the heating steam is given to the steam to be reheated flowing outside of the heat exchanger tubes 17, so that the heating steam is gradually cooled into a condensed state. That is, the heating steam thus cooled flows through the interior of the heat exchanger tubes 17 in the form of two-phase flow such as annular, wavy, and laminar flow condition. As a consequence, the heating steam, at the entrance portion of the heat exchanger tube 17, which has been in a vapor phase of a quality (wt % of vapor) substantially equal to 1 is changed into a quality substantially equal to 0 mostly composed of drain at the delivery portion of the tube 17. The steam mostly composed of drain is then sent through the low-temperature chamber 20 of the header 15 or 16 into the drain tank. A portion of the steam not condensed into drain is delivered outside through the vent steam outlet pipe 24.
It should be noted that the heating steam and steam flow through the first and second reheating devices in substantially the same manner. Heat exchanger tubes 17 of a low-fin type are ordinarily utilized because the heat transfer coefficient within the tubes 17 accompanying condensation phenomenon is higher than that of the exterior of the tubes wherein heat is transferred in a single phase of steam.
The flow condition of steam in each heat exchanger tube 17 is not always same as described above, but is varied depending on each tube. As shown in FIG. 4 the inlet and outlet ends of the U-shape tubes 17 are connected to the high-temperature chamber 19 and the low-temperature chamber 20, and the reheated steam outside the tubes 17 flows vertically upwardly relative to the heat-exchanger tubes 17. Accordingly, in the upper leg of a heat-exchanger tube 17a located at an outermost position among the bundle of the tubes 17, the heat duty becomes minimum, because the reheated steam outside that part of the tube 17a has been heated to a high temperature by the heat-exchanger tubes 17 below the upper leg of the tube 17a, and hence the temperature difference between the interior and exterior of the upper leg of the tube 17a becomes minimum. On the other hand, in the lower leg of the heat-exchanger tube 17a, heat is transferred between the heating steam within the tube 17a and reheated steam outside thereof which has not yet been heated by other heat exchanger tubes 17, and hence the temperature difference between the interior and exterior of the part of the tube 17a becomes maximum, and the heat duty also becomes maximum.
The flow rate of the heating steam flowing through the heat exchanger tube 17 is mostly determined by the heat duty in the tube, and therefore the distribution of the heating steam flowing through the tubes 17 must be reduced depending on the position of the tubes 17 from the outermost tube 17a toward the innermost tube 17b. However, the heat-exchanger tubes are all connected commonly between the high-temperature chamber 19 and the low-temperature chamber 20, and are thus subjected to the same pressure difference determined by the operating condition of the reheating device. As a consequence, the flow rate of the heating steam flowing through the heat exchanger tubes 17 is determined automatically by a flow resistance in the tubes 17 and the heat duty in these tubes 17.
In cases where the pressure difference between the high-temperature chamber 19 and the low-temperature chamber 20 is not high, a static pressure at a position in the lower leg of the outermost tube 17a, for example, tends to be made equal to that of the low-temperature chamber 20, and the flow of the heating steam in the tube 17a is blocked at that point. However, the heat-exchange caused thereafter in the tube part continuously produces drain which gradually fills the interior of the tube 17a at that position. The drain is further cooled by the reheated steam flowing outside of the tube in such an extent that the subcooling of the condensate of 50.degree. to 60.degree. C. is frequently caused.
Furthermore, in accordance with increase in drain filling interior of the tube, the heating surface is reduced as well as the quantity of steam flowing therethrough. The reduction of the steam flow reduces the two-phase flow resistance in the tube, thereby increasing the pressure applied across the drain-filled part, and exhausting the subcooled drain into the low-temperature chamber 20. Since a portion, from which the drain has been exhausted, provides a new heating surface, a large quantity of heating steam flows into the tube, thereby causing a hunting phenomenon. When the difference in heat duty between an outer tube such as tube 17a and an inner tube such as a tube 17b shown in FIG. 4 increases, the hunting phenomenon is exaggerated.
More specifically, the drain is caused to stay at an upstream side of the lower leg of the tube 17a, and the steam not yet condensed and held in the low-temperature chamber 20 flows back into the part of the tube on the downstream side of the drain staying portion. In this case, a periodic temperature variation occurs in a portion where the heat exchanger tube is welded to a tube plate 26, thus inevitably entailing a problem of thermal fatigue. The above described hunting phenomenon and the causing of fatigue in the welded part between the heat-exchanger tube and the tube plate 26, must be avoided for improving the reliability and safety of the control system of a nuclear power plant.
For overcoming the difficulty, there has been proposed a device wherein an orifice plate having a number of holes is secured to the tube plate, for instance by welding, while the holes are aligned with those of the tube plate, and the diameter of the holes are varied in accordance with the temperature distribution of steam flowing outside of the tubes 17. Although such a device is simple in construction and easy in realization, various difficulties have been encountered when it is used practically. Since the orifice plate has been welded directly to the tube plate, the removal of the orifice plate at the time of inspection and maintenance of the heat-exchanger tubes is not easy. Furthermore, gaps tend to be created between the orifice plate and the tube plate so as to provide leakage paths of steam by-passing some part of heat exchanger tubes 17, and making it difficult to guarantee appropriate operation of the orifice plate. In cases where a sufficient anti-corrosive property cannot be assured for the material of the heat exchanger tubes, the tubes tend to be corroded by vortices produced after the orifice plate.